Datasheet
S
e
=
(V
IN
± V
OUT
) x K
SL
+ V
SL
T
S
n
=
V
IN
x A x R
S
L
m
C
=
S
e
S
n
1
S x (m
C
± 0.5)
Q
=
Z
Z
=
1
C
OUT
x ESR
Z
n
=
S
T
Z
P
=
1
C
OUT
x
1
R
LOAD
+
1
K
m
x A x R
S
V
SL
=
I
OS
x T
C
RAMP
K
SL
=
g
m
x T
C
RAMP
K
m
=
(D ± 0.5) x A x R
S
x T
L
+ (1 - 2 x D) x K
SL
+
1
V
SL
V
IN
R
LOAD
A x R
S
x
x
1 +
s
Z
Z
x
=
V
OUT
V
COMP
1 +
s
Z
P
+
1 +
s
Z
n
x Q
s
2
Z
n
2
1 +
R
LOAD
K
m
x A x R
S
1
RAMP
10 nF
LO
-VCC
C
RAMP
R
RAMP
10 nF
1N914
R
RAMP
=
VCC ± 0.5V + V
RAMP
25 PA - I
OS
RAMP
VCC
C
RAMP
R
RAMP
R
RAMP
=
VCC - V
RAMP
I
OS
- 25 PA
LM5116
www.ti.com
SNVS499G –FEBRUARY 2007–REVISED MARCH 2013
(40)
Figure 39. R
RAMP
to VCC for V
OUT
> 7.5V
For V
OUT
< 7.5V, a negative VCC is required. This can be made with a simple charge pump from the LO gate
output. Install a resistor from the RAMP pin to the negative VCC.
(41)
Figure 40. R
RAMP
to -VCC for V
OUT
< 7.5V
If a large variation is expected in VCC, say for V
IN
< 11V, a Zener regulator may be added to supply a constant
voltage for R
RAMP
.
MODULATOR TRANSFER FUNCTION
The following equations can be used to calculate the control-to-output transfer function:
(42)
(43)
(44)
(45)
(46)
K
m
is the effective DC gain of the modulating comparator. The duty cycle D = V
OUT
/ V
IN
. K
SL
is the proportional
slope compensation term. V
SL
is the fixed slope compensation term. Slope compensation is set by m
c
, which is
the ratio of the external ramp to the natural ramp. The switching frequency sampling gain is characterized by ω
n
and Q, which accounts for the high frequency inductor pole.
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